A variable gain amplifier (VGA) is an amplifier that has a signal gain (Av) determined by a control input. VGAs are utilized in communication systems and other electronic systems where an input signal level varies over a wide dynamic range. The VGA is often embedded inside of an automatic gain control (AGC) feedback loop which attempts to maintain the output signal level nearly constant even though the level of the input signal varies over time. When operating in a feedback loop, a VGA with exponential gain control characteristics is usually desirable for its large dynamic control range, constant loop settling time independent of absolute gain, and the ability to use the control signal as a logarithmic received signal strength indicator. It is also typically important to minimize the variation in the gain slope over the control input range because this factor is an important one in AGC loop stability. The control input can be discrete and digital in nature, such as an N-bit binary word. In this case, accurate exponential gain control can be accomplished by switching in or out gain control elements. In situations where the gain control loop is continuous and analog in nature, then either a voltage or current can be used to control the gain. Achieving accurate exponential gain control for such continuous gain control loops, however, is not easily achieved with CMOS circuitry.
Some prior VGAs have attempted to solve this problem by providing multiple gain stages. A limited range of the gain response for each stage can be used that are more linear in nature with respect to a logarithmic scale or log-linear. By serially combining multiple gain stages and controlling them individually, these prior VGAs could achieve a gain response that more closely tracked an ideal log-linear response than if a single stage were utilized. However, these prior solutions are still attempting to fit as best possible a fundamentally non-log-linear gain response curve for each stage onto a desired log-linear response.